Consider a canonical thermodynamic system that consists of a gas in a cylinder with a moveable piston. Help me draw a picture on the board.

SWBQ: On your small whiteboard, tell me a quantity you can measure for this system.

Students should generate the following. For each, follow up with: How would you measure _ for this system? Then label them the picture, along with a way to measure that quantity:

Volume ($V$) - It can change if the piston moves and you can measure height with a ruler.

Temperature ($T$) - Put a thermometer in it!

Pressure ($p$) - If in equilibrium, $F_{net} = 0$, so if we account for the mass of the piston and the external pressure we can determine the pressure. (What happens if there's disequilibrium?)

Follow-up: How would you hold each of these variables constant?

There is an analogy between the gas in a piston and the PDM. With your group, discuss what is analogous between the two systems.

Good questions for helping students:

How do you change quantities in the gas?

How do you change quantities in the PDM?

How do you hold quantities constant?

Are there similarities in the dimensions of quantities between the two systems?

Final dictionary:

$p \rightarrow F_L$

$V \rightarrow x_L$

$T \rightarrow$ ? (students may not have enough information, but it should go to $F_R$.)

We are probably going to need a fourth variable here. It turns out this fourth variable is Entropy ($S$), which you will learn about more during the rest of the course. This is a good place to note that holding entropy constant is equivalent to insulating the system and remarking on how this is different from holding temperature constant.